3.1. Degradation of Nitrogen Mustard
Since, HN3 undergoes spontaneous hydrolysis in water [
25], at the beginning of our experiments the hydrolysis the hydrolysis rate of prepared HN3 was studied. The GC-FID analysis confirmed that hydrolysis of HN3 fulfil the first order kinetics equation [
24]. Therefore, for analysing the kinetics data of HN3 hydrolysis, an Equation (1)
was used, where c
t denotes the residual concentration of HN3 in time t and c
0 stands for the initial concentration of HN3. The rate constant of the spontaneous hydrolysis of HN3 was at pH 6 determined to be 0.029±0.008 min
−1 which is in line with previously reported results [
25].
After these initial experiments the oxidation power of ferrate(VI) towards HN3 was tested. The degradation of the nitrogen mustard HN3 was studied as a function of the concentration of remaining nitrogen mustard HN3 in a solution and time at various pH (
Figure 2).
As expected, after the addition of purple ferrate Fe(VI) solution to a reaction mixture, the colour changed and a brown precipitate containing iron(III)/iron(II) products as the final species was formed [
26]. It can be supposed that oxidation of HN3 by ferrate(VI) is 2e
- transfer step (Fe
VI→Fe
IV→Fe
II) rather than 1e
- transfer step, as suggested by previous reports concerning reactions of iron(VI) with amines [
5,
27,
28].
The obtained data were studied using several kinetics models. Based on our measured kinetic data of the decomposition of HN3 using Fe(VI) it follows that the best fit was found a pseudo-second order kinetics model. This observation is consistent with previous results, where the kinetics of amine oxidation by ferrate(VI) was monitored [
5,
27,
28]. Therefore, the following equation was applied to calculate the second-order rate constant k
2 for all degradation processes at all pH used:
where c
e is an equilibrium concentration of HN3. The rate of HN3 degradation is represented by kinetic curves in
Figure 3.
As can be seen from
Figure 3 the complete degradation of HN3 by ferrate(VI) was reached after ca 4 min in all solutions of different pH. All the curves that were calculated according to Equation (2) fit the experimental points well, while the individual statistical parameters are summarized in
Table 1. This confirms the correctness of our reasoning that it is a reaction with pseudo-second order kinetics.
In the following
Figure 4, the bar diagrams of calculated rate constants k
2 according to the Equation (2) at different pH are plotted.
From
Figure 4 it is easily visible that the decomposition rate of HN3 by ferrate(VI) strongly depends on the pH of the solution. The highest rate was observed for pH=4, followed by pH=5 and pH=3. The lowest rate was observed for the pH=6. The lower decomposition rate of nitrogen mustard at pH=3 compare to pH=4 is probably caused by the strong self-decomposition reaction of ferrate(VI) to Fe(III)/Fe(II) by water via the intermediary of Fe(IV) and Fe(V) species, which could reduce the efficiency of the degradation process itself. At pH=3 H
2FeO
4 is mainly present due to the pKa = 3.5 ± 0.2 [
6,
29,
30]. At this acidic pH the formation of a diferrate(VI) with fast intramolecular oxo-coupling producing O
2 and diferryl(IV) species was proposed. However, the authors of this study point out that in the acidic solutions, the kinetics of ferrate(VI)-mediated water oxidation (ferrate decomposition) becomes a complex problem. Similarly to our study with HN3 a second-order decay process can be resolved. However, the self-decomposition of ferrate(VI) is faster than oxidation of nitrogen mustard with condensation and dimerization of monomeric ferrate as the rate-determining step [
29]. In the following
Table 1, the calculated values of all rate constants are summarised.
It can be easily seen that the rate of the decomposition of HN3 by ferrate(VI) is at the same pH levels approximately two orders of magnitude higher than that of spontaneous hydrolysis of HN3. This means that the impact of hydrolysis on the decomposition of HN3 is very low compare to ferrate(VI) and can be during calculations neglected. In addition, this means that Fe(VI) is a strong enough oxidizing agent against nitrogen mustard even at an almost neutral pH, which in field conditions means simplification of handling during eventual decontamination of surfaces, people, vehicles, etc. ferrate(VI) solution.